The Ancient Egyptians didn't just mummify their dead human ancestors, they also mummified animals - in their millions. It's possible up to 70 million animals were mummified and then buried in underground catacombs at over thirty sites across Egypt!
I'm by no means an expert, but an Egyptian Archaeology professor I once had mentioned that there are multiple theories regarding this. The most interesting one was that massive amounts of animals were mummified in order to be a "food source" in the afterlife.